This invention relates to a step-index type optical fiber for optical communication which is prepared from multicomponent glass.
This optical fiber is formed of core glass and cladding glass having a smaller refractive index than the core glass. Optical communication glass fiber is designed to conduct light information from one end to the other in a state completely enclosed in the core glass by utilizing the total reflection of light at the boundary between the core glass and cladding glass.
The above-mentioned type of optical fiber is demanded to meet the requirements listed below:
(1) Fiber, particularly cladding glass should have a high resistance to chemicals, such as water, acids, alkalis, and a high resistance to weathering, in other words, high durability. PA0 (2) Neither core glass nor cladding glass should crystallize during manufacture. This requirement is indispensable. PA0 (3) The core glass should minimize loss of light transmitted therethrough. Transmission loss of light should be smaller than 20 dB/km as measured from the whole fiber. Where fiber glass has a high melting point, impurities tend to be carried into the glass from, for example, a crucible. As a result, scattering of light undesirably arises in such defective glass, leading to higher transmission loss of light. PA0 (4) The core glass should have a thermal expansion coefficient larger than that of the cladding glass by a difference .DELTA..alpha. of less than 3.times.10.sup.-6 cm/cm .degree. C. (at 0.degree. to 300.degree. C.). Where both thermal expansion coefficients have an unduly large difference, then the resultant fiber glass will decrease in reliability. PA0 (5) The core glass should have a refractive index (n.sub.1) larger than that of the cladding glass (n.sub.2) by a larger ratio .DELTA.n (=n.sub.1 -n.sub.2 /n.sub.1) than 0.003. PA0 (1) SiO.sub.2 is a network former of fiber glass. Where the content of SiO.sub.2 falls below 50% by weight, the resultant glass decreases in resistance to water. Where the content of SiO.sub.2 rises above 80% by weight, the resultant glass fails to have a desired large refractive index. For elevation of resistance to water, it is preferred to use more than 60% by weight of SiO.sub.2. Part of SiO.sub.2 may be replaced by GeO.sub.2 to control the viscosity of a core glass member at a temperature of fibrous drawing. In this case, however, the content of GeO.sub.2 is restricted to 30% at maximum (at this time the content of SiO.sub.2 ranges between 20 and 50% by weight). Where the content of GeO.sub.2 increases over said maximum level, the viscosity of the core glass at a temperature of fibrous drawing excessively falls, presenting difficulties in the fibrous drawing of the resultant core glass. PA0 (29 Al.sub.2 O.sub.3 is an important ingredient to elevate the water resistance of glass and minimize its tendency toward crystallization. A smaller content of Al.sub.2 O.sub.3 than 0.5% by weight can not provide glass having a high resistance to water and a small tendency toward crystallization. Where, however, the content of Al.sub.2 O.sub.3 exceeds 7% by weight, then the core glass is ready to crystallize during the fibrous drawing. The content of Al.sub.2 O in the core glass member is preferred to range between 1 and 6% by weight. PA0 (3) Alkali metal oxides used with an optical fiber embodying this invention chiefly include Na.sub.2 O, K.sub.2 O and Li.sub.2 O. It will well serve the purpose, if the glass contains at least one of these components or a mixture thereof. The alkali metal oxide is a glass network modifier. As the alkali metal oxide is used in a larger amount, the resultant glass tends to have a lower resistance to wear. An upper limit to the content of an alkali metal oxide in the core glass is chosen to be 23% by weight. Where, however, the content of an alkali metal oxide falls below 10% by weight, then the core glass tends to crystallize during the fibrous drawing. Therefore, the content of an alkali metal oxide in the core glass is preferred to range between 12% and 21% by weight. The percentage of the above mentioned Na.sub.2 O, K.sub.2 O and Li.sub.2 O can be properly adjusted within said range, depending on the property demanded of the core glass. PA0 (4) CaO contributes to the elevation of the water resistance and refractive index of glass. Where, however, CaO is contained in a larger amount than 10% by weight, the resultant glass is likely to crystallize. The content of CaO in the core glass is preferred to range between 2 and 10% by weight. PA0 (5) MgO restricts the occurrence of haze in glass resulting from exudation of alkali components, and consequently improves resistance to weathering. Where, however, the content of MgO increases over 5% by weight, the resultant glass is ready to crystallize. The content of MgO in the core glass is preferred to range from 1 to 5% by weight. PA0 (6) B.sub.2 O.sub.3 is effective to prevent glass from crystallization and decreasing in viscosity at high temperature. Where, however, the content of B.sub.2 O.sub.3 increases over 15% by weight, the resultant glass unduly drops in viscosity at a temperature of fibrous drawing, presenting difficultiesin carrying out said fibrous drawing. Where the content of B.sub.2 O.sub.3 in the core glass favorably ranges between 3 and 12% by weight, then the resultant core glass is saved from crystallization, making it possible to manufacture an optical fiber with a high reproducibility. PA0 (1) SiO.sub.2 is network formerof fiber glass. Where the content of SiO.sub.2 falls below 60% by weight, then the resultant cladding glass decreases in resistance to water. Where the content of SiO.sub.2 rises above 80% by weight, then the resultant cladding glass has an increased viscosity at high temperature, undesirably showing a larger viscosity difference between the core glass and the cladding glass. The content of SiO.sub.2 in the cladding glass is preferred to range between 65 and 75% by weight. PA0 (2) Al.sub.2 O.sub.3 is effective to elevate the water resistance of the cladding glass and minimize its tendency toward crystallization. Where Al.sub.2 O.sub.3 is applied in a smaller amount than 0.5% by weight, the resultant cladding glass fails to have a satisfactory property. Conversely, where the content of Al.sub.2 O.sub.3 exceeds 10% by weight, then the resultant cladding glass tends to crystallize during the fibrous drawing. The content of Al.sub.2 O.sub.3 in the cladding glass is preferred to range between 2 and 8% by weight. PA0 (3) Alkali metal oxides chiefly includeNa.sub.2 O, K.sub.2 O and Li.sub.2 O. It will well serve the purpose, if the cladding glass contains at least one of these components, or a mixture thereof. Since the cladding glass is demanded to have a higher durability than the core glass, the content of an alkali metal oxide in the cladding glass is controlled to be smaller than when applied in the core glass. It has been discovered that where the content of an alkali metal oxide rises above 17% by weight, the resultant cladding glass fails to have a sufficient resistance to water, and that where said content falls below 9% by weight, the resultant cladding glass is ready to crystallize during the fibrous drawing. The content of an alkali metal oxide is preferred to range between 9 and 16% by weight. PA0 (4) CaO contributed to the elevation of the water resistance of the cladding glass. Where, however, CaO is used in a larger amount than 5% by weight, then the resultant cladding glass is likely to crystallize. PA0 (5) MgO is effective to suppress the occurrence of haze in the cladding glass and elevate its resistance to weathering. Where, however, the content MgO increases over 4% by weight, the resultant cladding glass is ready to crystallize. The content of MgO in the cladding glass is preferred to range between 0 and 3% by weight. PA0 (6) B.sub.2 O.sub.3 contributes to the elevation of water resistance of the cladding glass and its refractive index. Where, however, the content of B.sub.2 O.sub.3 exceeds 15% by weight, then the resulting cladding glass has too high a refractive index. PA0 (7) Even a minute amount of ZnO, ZrO.sub.2 and TiO.sub.2 is very effective to increase the water resistance of the cladding glass. Where, however, the content of any of these components increases over 7%, then striae tend to be grown in the resultant cladding glass. The content of any of said oxides favorably falling within the range of 0.5 to 3% by weight is sufficiently effective to increase the water resistance of the resultant cladding glass.
Improvement on the known optical fiber has been primarily intended to decrease loss of light transmitted therethrough. It is reported that an optical fiber has been developed in which transmission loss of light is of the order of about 5 dB/km. Though favored by a small transmission loss of light, the optical fiber developed to date has the drawback that it is unsatisfactory in respect of resistance to chemicals and weathering. An optical fiber is generally demanded to have a long effective life. However, the above-mentioned prior art optical fiber, though improved in transmission loss of light, has insufficient durability and consequently low reliability, failing to be put into practical application.
A known multicomponent optical fiber favored by a low transmission loss of light is formed of a core glass member prepared from 20% by weight of Na.sub.2 O, 9% by weight of CaO and 71% by weight of SiO.sub.2 and a cladding glass member prepared from 22% by weight of Na.sub.2 O, 3.5% by weight of CaO and 74.5% by weight of SiO.sub.2. Though appreciably reduced in transmission loss of light, this optical fiber, particularly, the cladding glass member thereof contains a large amount of alkali. This alkali component tends to exude to the surface of the fiber and decrease its durability.
A step-index type optical fiber is generally manufactured by causing molten core glass and cladding glass simultaneously to flow downward respectively through the bottom openings of the inner and outer unit crucibles constituting a double crucible assembly, and later drawing both falling glass materials into a fine fibrous form. These core and cladding glass materials are preferred to have a viscosity ranging from 10.sup.5 to 10.sup.3 poises during the rundown through the bottom opening and subsequent drawing in the fibrous form. If the glass material crystallizes during the fibrous drawing, it gives rise to increased transmission loss of light and decreased tensile strength. Therefore, it is an indispensable requirement for the glass material to be saved from crystallization during the fibrous drawing.